These are awesome. I have one for my point and shoot casio and it is really fun when I go to central america. I looked into getting one for my 30D but it cost more than my camera. I'm sure prices have come down now since though.
Well, beyond just the watertight concerns, the front element will be fogged and/or vignetted unless the bag is pushed up right onto the front lens element. I don't see any way of keeping it there unless the lens happens to be just the right length, either.
I think I will let someone else give it a go first.